1. Field of the Invention
This invention relates to nonlinear optical systems containing 3-methyl-4-methoxy-4'-nitrostilbene, which systems are capable of second harmonic generation (SHG), electro-optic modulation, and other useful nonlinear optical and electro-optic effects
2. Description of Related Art
The nonlinear optical response of a molecule can be described by the following expansion: EQU .mu.=.mu..sub.0 .alpha..sup.(1) E+.beta.EE+.gamma.EEE+ . . .
where .mu. is the induced dipole moment and .mu..sub.0 is the permanent dipole moment of the molecule; .alpha., .beta., and .gamma. are tensors representing the linear, second order and third order polarizabilities, respectively; and E is the local electric field. The induced polarization of an ensemble of molecules, such as a crystal, can be described by the following equation: EQU P=P.sub.0 +.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+ . . .
where P is the induced polarization and P.sub.0 is the permanent polarization; .chi..sup.(1), .chi..sup.(2) and .chi..sup.(3) are tensors representing the linear, second order and third order susceptibility, respectively; and E is the applied electric field. Second order nonlinear optical phenomena such as second harmonic generation, sum and difference frequency generation, parametric processes and electro-optical effects all arise from the .chi..sup.(2) term.
To have a large .chi..sup.2, a molecule should both possess a large .beta. and crystallize in a noncentrosymmetric structure. Centrosymmetric crystals have vanishing .chi..sup.(2) and are therefore incapable of second harmonic generation.
Franken et al., Physical Review Letters, Vol. 7 118-119 (1961), disclose the observation of second harmonic generation upon the projection of a pulsed ruby laser beam through crystalline quartz. They observed the generation of the second harmonic of light, in which light of 6943 .ANG. was converted to light of 3472 .ANG.. The use of a laser remains the only practical way to generate an E large enough to be able to detect the SHG phenomenon.
Coda et al., J. Appl. Cryst., Vol. 9, 193 (1976), disclose SHG in a powder sample of 4-methoxy-4'-nitrostilbene.
Useful reviews of the art relating to nonlinear properties of organic materials are given in the following references: "Nonlinear Optical Properties of Organic and Polymeric Materials", D. J. Williams, ed., American Chemical Society, Washington, D.C. pp. 57-80 (1983);
D. J. Williams, Angew. Chem., Int. Ed. Engl., Vol. 23, 690 (1984); "Nonlinear Optical Properties of Organic Molecules and Crystals", Vol. 1, D. S. Chemla, et al., ed., Associated Press, Orlando, Fla., pp. 227-296 (1987).
Ruby, U.S. Pat. No. 3,514,495, discloses a process for the preparation of 4-nitrostilbenes by the condensation of 4-nitrotoluenes with certain aromatic aldehydes. The compound which forms the basis of the present invention, 3-methyl-4-methoxy-4'-nitrostilbene, is not disclosed.
Although a large number of organic and inorganic materials capable of SHG have been found since the Franken et al. discovery, an intense search continues for new materials.